


Taking the plunge

by caprigender



Category: Undertale (Video Game)
Genre: F/M, Reader-Insert, reader has a vagina no mention of breasts
Language: English
Status: Completed
Published: 2015-11-30
Updated: 2015-11-30
Packaged: 2018-05-04 05:15:04
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 1,769
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5321807
Author URL: https://archiveofourown.org/users/caprigender/pseuds/caprigender
Summary: <blockquote class="userstuff">
              <p>Undertummy smut based off a post on tumblr that got a non-zero number of notes so im actually posting it.</p>
            </blockquote>





	Taking the plunge

Sans’ magic crackled and pulsed, strangely solid under your fingertips. The glowing blue flesh (was that really what it was? You couldn’t be sure what to call anything anymore) was soft, cool, and mostly translucent. The white bones beneath it were tinted aqua blue and distorted as if you were looking at them through rushing water. You ran your hands across Sans’ belly. It still didn’t look quite real to you, like any moment it might dissolve into thin air. You’d long since learned that the magic monsters were made up of was a lot more solid than it looked. 

You tightened your grip on the softly glowing bulge and watched Sans’ face twitch in reaction. He groaned and let his skull drop back against the pillows while you continued your curious examination of his body. The buzzing against your palms was intoxicating and you leaned in to draw your tongue across his skin. Sans let out a breathy whimper. The magic felt just as strange against your tongue as it did on your fingers. It tasted metallic, like licking a battery, but the taste was also strangely sweet with some flavor you couldn’t exactly recognize.

You sat back on your heels and tugged your shirt up over your head. You could take your time later to figure out what exactly Sans tasted like. Your first item of discovery was learning what he felt like when you pressed as much of yourself against him as possible.

Sans watched you undress hungrily. His eye glowed in his left socket with unearthly intensity. You could feel his gaze burning across your skin. You tried to take it slowly, maybe tease him a little bit as you removed your pants and undergarments, but the adrenaline rushing through you left you shaky and desperate.

“Wow,” Sans breathed in a hushed whisper. Your heart jumped in your chest and you felt heat rush to your face. “You sure you still wanna be here? Not too late to make like a trampoline and bounce out.”

You frowned. It might have been a joke but the self-deprecating humor never really struck you as very funny. “Lucky I don’t need a trampoline to jump your bones.” You shot back with a wink.

Sans chuckled, “Go right ahead.”

You managed to keep your hands from shaking as you slid them up his sides. You didn’t exactly end up jumping on him as much as crawling up his body, making sure to take your time and enjoy it. The magic of his stomach rubbed against you. The humming energy sent vibrations across your skin. You settled down straddling his pelvis, his stomach pressing against yours, your chest forming to the cold ridges of his ribcage. An electric blue tongue flicked out from between his teeth for a moment. You smiled and pressed a gentle kiss against his grin.

You tilted your hips to grind against him. A soft moan escaped your lips at the friction of his magic against your lower abdomen. Sans groaned and arched his back, pressing his pubis against your lips, already slick in anticipation. You tucked your head in the gap between his neck and collar bone. His vertebrae felt cool and rough on your cheek and you closed your eyes as you moved against him. Pressed this close you could hear every whimper, grunt and moan that passed through the skeleton’s tightly clenched teeth. He gripped you with cold bony fingers, filling his hands with as much of you as he could hold. You pressed down against him, rubbing your clit against his bones which were now warm with friction and your own body heat.

Sans gasped and groaned “Don’t hate me for this, but I really want to be inside you right now.”

You giggled and traced along the line of his jaw with your tongue. “That sounds like a lot of fun.” The bone under your clit began to crackle with the same electric blue energy lighting up his eye socket. You whimpered into his neck at the vibrations and tingling pressure of Sans’ dick growing up against you. You sat up and leaned back to watch as the member finally finished forming, standing erect just above his pubic symphasis. You took a moment to admire the glowing blue dick, teasing it with slow shallow movements. Sans sighed in pleasure. You reached over to get a palmful of lube from the bottle on the bedside table. The light in his eye socket followed the path of your hand. His breath was steady but shallow. You reached down to slick him up in a few quick strokes, focusing on the way his body twitched under your hands.

You positioned yourself on top of him and slowly slid down his shaft. His magic pulsed inside you, stretching you out and sending spikes of pleasure shooting up your spine, through your chest, down your legs to curl your toes. His eyes rolled back in his skull and he let out a throaty groan, shifting his pelvis up to meet yours. You pulled back and settled down on him again. Your ass pressed up against the hard ridges of his femurs. You hummed in satisfaction as you started moving faster on top of him. The deeper he was inside of you the harder it was to feel the buzzing of his magic, but you were more than able to make up for it once you got your momentum going. You bit your lip to stifle a moan as you slammed back down against him. Your skin made a slapping sound against his bones and you made a mental note not to get too rough and bruise yourself against him.

Your hands gripped his stomach, the soft pulsing magic was so different from the hard bones you were grinding against. It was still cool despite the friction and the sweat beading on Sans’ skull that said he was overheating. He looked up at you with eyes half lidded and you knew that wasn’t physically possible, but technically nothing about that night was supposed to be physically possible. You were fucking a skeleton and not about to get hung up on all the idiosyncrasies of the scenario. Hell, if you started questioning something too much it just might disappear and the last thing you wanted while riding dick into oblivion was for your partner to vanish and leave you feeling sticky and unfulfilled.

You rocked back and forth on his cock, using the springy momentum of the bed to fuck down on him faster. His belly shifted along with your rhythm. It slapped up against you with each thrust and you found yourself leaning into its soft squish.

Sans gasped, “Heh Heh, someone seems eager,”

You grinned down at him. He always got sassy when he was close to coming in some snarky attempt to keep his composure. It rarely worked. “You maybe wanna put in some effort, lazy bones?”

“Why should I? You’re doing such a good job already,” his lazy grin widened into a smirk, “Bet you can’t fuck me harder.”

It was a cheap trick and you knew it, but it worked like a charm and you doubled down your efforts. Leaning heavily against his belly for support, you thrust down on him as fast as you could. The pressure and fruction built up inside you until you were almost bursting, His hands flew to your ribcage, trying to pull you closer. The cold phalanges dug into the flesh of your sides. He was close. Your face flushed with pride and lust at his quiet cries and soft grunts. Your legs were exhausted and your breathing heavy but you just had to hold on a bit longer and he would…

Sans shivered and suddenly you felt yourself pitch forward. Your hands felt as if they had been plunged into a bucket full of aloe vera and you were now elbows deep in Sans’ stomach. His eyes flew open and he cried out.

You yelped and pulled back, scrambling off him to the edge of the bed. Sans reached out to grab you just in time to be pulled over and onto the floor in a pile of limbs and blankets.

The two of you sat there, sticky and gasping for breath. Your heart was pounding in your ears and your hands were trembling. You looked at them, noticing the trace amounts of glowing blue magic under your finger nails. You shuddered as you watched it evaporate.

“Are you ok?” You turned to Sans who looked just as shaken up as you felt. His chest heaved with his breathing and sweat dripped from his brow. You noticed the lighter teal splatters on his pelvis and your thighs and realized he must have come sometime during all that commotion.

“I think I’m fine,” you responded, “Are you fine? I didn’t hurt you, did I?”

He shook his head, “No, that was… well… that was definitely different.”

You chuckled nervously. Different. A wave of relief crashed over you and your nervous giggling turned into a fit of laughter you couldn’t stop. You leaned on Sans, resting your forehead against his collar bone and gasping for breath as you shook uncontrollably.

“Hey, are you sure you’re ok?” He asked, “You’re kind of cracking up on me here.”

“S-sorry” you cackled, wiping tears from the corners of your eyes, “I was so scared that I had hurt you or something.” You clutched your stomach and tried to control your breathing through the giggles.

“Oh god,” he chuckled, “don’t worry. That was a lot of things but uh… It definitely didn’t hurt.”

You turned to him with a predatory grin that might have been ruined by the last lingerings of your laughing fit. “You saying you wanna try that again sometime? You like having me inside you?”

Sans’ shocked speechless face was priceless, all wide eyed and slack jawed and it kinda made you want to eat him up. He turned to avoid your gaze as his cheekbones lit up in a bright blue blush. “Heh, I might be saying that.” He let out a sigh and collapsed back on the floor. “That was intense.”

You hummed in agreement and settled down next to him. “You wanna move this party back up to the bed?”

He shrugged and tugged you closer. “I dunno. Seems pretty perfect down here,” he whispered against your neck.

You smiled. You were sticky and damp and the floor was cold and hard, but you had to agree. Nestled in Sans’ arms, this all seemed pretty perfect.


End file.
